PRU - GPIO Toggle

The PRU controls the GPIO output by writing to R30. The PRU configures the pin mux registers for PRU_R30[30, 31] outputs as an initialization step. The example toggles bits 30-31 of R30 by inverting each bit. The ARM compares the original and final values of R30 to verify that bits 30 and 31 were toggled. The PRU then resets the pin mux registers to their original values.

You can find a more detailed description here: http://processors.wiki.ti.com/index.php/PRU_Linux-based_Example_Code